Fluids, such as carbon dioxide (CO.sub.2) gas, often contain an amount of a reactive sulfur compound, such as a carbonyl sulfide (COS), a hydrogen sulfide (H.sub.2 S), mercaptans (RSH), or combinations thereof, which is undesirable. Carbon dioxide gas, nitrogen gas, natural gas liquids, and other fluids contaminated with reactive sulfur compounds are often commercially unacceptable and cannot be sold until the contaminant sulfur compounds have been substantially removed. In fact, most commercial purchasers of natural gas or other fluids or gases, for example, will not purchase gas from the gas producer unless the contaminant sulfur compounds are below a certain specified number. This is because when gasoline, for example, is formed, synthesis catalysts are used; however, synthesis catalysts are extremely sensitive to sulfur. As such, it is desired to remove the contaminant sulfur compounds from the various fluids such as carbon dioxide (CO.sub.2) gas.
It is known that H.sub.2 S and RSH can be readily removed from different types of fluids by passing such fluids through, for example, an iron oxide bed that is reactive with H.sub.2 S and RSH. However, COS is generally not very reactive with the iron oxide bed materials or other metal oxide materials. When fluids contaminated with COS are passed through a metal oxide bed, generally very little, if any, of the COS will be removed from the CO.sub.2 gas or other fluid, especially at temperatures below 3000 F. This is problematic because not only is COS a contaminant, but with the passage of time the COS will hydrolyze into H.sub.2 S, which is considered a highly undesired sulfur contaminant. Thus, it is desired to have a method or composition for removing COS from a fluid because COS is considered a contaminant and because COS will convert to H.sub.2 S which is considered a particularly undesirable contaminant.
The conversion rate of COS to H.sub.2 S can be very slow, often taking at least a few weeks. It is known that COS can be converted to H.sub.2 S if an elevated temperature above ambient temperatures is used. As such, typically COS is removed from a fluid by using a high temperature removal process, with such removal process generally being difficult to perform and expensive to use. Even with the known high temperature removal processes, prior removal methods have not been very successful in removing a sufficient amount of COS from a fluid. Thus, it is desired to have a method that is less expensive, does not involve elevated temperatures, can be performed under ambient conditions, will cause the conversion in a short period of time, and that is easy to perform.
A method for removing COS under ambient conditions was disclosed in U.S. Pat. No. 4,290,879 ('879 patent). The method disclosed mixing a fluid stream with methanol, followed by passing the stream through a bed of potassium hydroxide (KOH). While this method worked it suffers from a couple of different problems. First, the use of methanol can be dangerous because it is highly flammable. This method involves the use of a hazardous compound, methanol, which is undesired. Also, the method requires the formation of a KOH bed which is typically not used in the sulfur removal industry. As such, the costs arc increased in this method because a bed that is not typically used must be formed. For these reasons it is desired to have a method that is not very hazardous, is economical, and that can be used with existing sulfur removal equipment. It is further desired to develop a method that does not always require a second treatment step, meaning a method whereby the COS is not converted to another contaminant that must then be removed. Instead it is desired to have a method whereby on occasion the COS is simply eliminated without a further step required to remove a newly formed contaminant.
Another method for removing COS from a fluid stream involves catalyzing COS to H.sub.2 S using a cobalt-molybdenum catalyst. The cobalt-molybdenum catalyst is operated at a temperature ranging between 500.degree. and 750.degree. F., a pressure ranging between 100 and 500 psig, and a space velocity ranging between 500 to 1.500 cubic feet per hour. After the reaction occurs, then the fluid must be passed through a metal oxide in order to remove the H.sub.2 S, which has been catalyzed from the COS. This method suffers from a number of problems, including the fact that cobalt is used, which is considered an extremely hazardous compound. Elevated temperatures are used which is also undesirable. Additionally, this method can be cost prohibitive.
As can be seen, it is difficult to thoroughly and inexpensively remove COS from fluids. Thus, it would be desirable to have a cost effective and efficient system for removing COS from various fluids, especially CO.sub.2 gas, without having to heat the fluids or any other part of the reaction system. It is also desired to have a safe and economical method for removing COS from fluids.